1. The Field of the Invention
The present invention relates to semiconductor devices. More particularly, the present invention relates to semiconductor gettering structures and methods of forming them. One embodiment of the present invention relates to ion-implanted gettering structures that are implanted substantially below the bottom of isolation trenches.
2. The Relevant Technology
In the microelectronics industry, a substrate refers to one or more semiconductor layers or structures which includes active or operable portions of semiconductor devices. In the context of this document, the term xe2x80x9csemiconductive substratexe2x80x9d is defined to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive material such as a semiconductive wafer, either alone or in assemblies comprising other materials thereon, and semiconductive material layers, either alone or in assemblies comprising other materials. The term substrate refers to any supporting structure including but not limited to the semiconductive substrates described above.
The ever-present pressure upon the microelectronics industry to shrink electronic devices and to crowd a higher number of electronic devices onto a single die, called miniaturization, has required development of isolation trenches to replace such structures as local oxidation of silicon (LOCOS) regions. As miniaturization continues to shrink dimensions of electronic devices, limitations on the ability to restrict chemical impurities in the fabrication process are being reached. To compensate for such chemical impurities limitations, gettering structures are formed within semiconductive devices that have an affinity for chemical impurities.
One prior art method of forming a gettering structure is blanket implantation of ions within the semiconductive substrate as illustrated in FIG. 1 so as to damage the material of the semiconductive substrate. In FIG. 1 it can be seen that a semiconductor structure 10 includes a substrate 12, preferably composed of a semiconductor material, such as silicon or germanium, and having an upper surface 26 and an oxide layer 14 thereupon. Blanket implantation of semiconductor structure 10 is illustrated wherein a gettering structure 32 comprises damaged silicon created by ions that have been implanted with mega electron volt (MeV) implantation equipment. As referred to herein, KeV implantation equipment can implant with energy in a range from about 25 Kev to about 600 Kev, and MeV implantation equipment can implant with energy in a range from about 25 Kev to about 2800 Kev. Damage is contained within gettering structure 32 which is at a depth d. Blanket implantation, however, causes damage in the semiconductive materials within gettering structure 32. A diode junction 42 is seen below region 40 which adjacent to active area 20 and field oxide region 15 which can be formed by LOCOS processing.
Gettering structure 32 includes substantial disturbance of the monocrystalline lattice of the semiconductive material that not only compromises the semiconductive integrity of semiconductor substrate 12, but also allows contaminants to migrate and be captured within gettering structure 32. Attempts to improve the gettering efficiency of gettering structure 32, such as heat treating, may be constrained by the allowable thermal budget in a given process.
Lattice vacancies and crystal originated pits (COPs) often form during the crystal pulling process. COPs both disturb the integrity of a semiconductor device due to surface pits. Self implantation of interstitials into region 30 seen in FIGS. 1 and 2, such as with silicon, can reduce the size of the surface pits or eliminate them with an anneal adequate thermal cycle.
Another way to form gettering structure 32 is to use a mask 18, as seen in FIG. 2, to keep the gettering implanted materials away from regions that will cause leakage or other problems, for example at an N-well edge. Mask 18, however, adds cost to the process. FIG. 2 illustrates the prior art method of implantation of a gettering structure 32 through a mask 18. It can been seen that a semiconductor structure 10 includes a substrate 12 composed of a semiconductor material such as silicon or germanium, an oxide layer 14, and mask 18. It can been seen that mask 18 has been patterned to form an implant corridor 22 through which ions may be implanted into substrate 12. Also shown is a bottom 38 of active area 20 below mask 18. A detriment of the structure see in FIG. 2 is the capital and energy requirements in masking the structure prior to implanting gettering structure 32.
Another problem of forming a gettering layer is the energy and equipment cost of using KeV implantation equipment versus MeV implantation equipment. KeV implantation equipment, which implants close to the surface of implantation, incurs a greater risk of defects close to the surface of implantation. Conversely, MeV implantation equipment can implant farther away from the surface of implantation than KeV implantation equipment and therefor has less of a risk of defects close to the surface of implantation. The cost of KeV implantation equipment is less than that of MeV implantation equipment.
Gettering structure 32 has a shape that has an initial width equivalent to the width of an implant corridor 22 and a final Width W that is formed naturally by spreading implanted species. Region 30 may cause contaminants to be more mobile through the damaged lattice therein. Gettering structure 32 is required to be formed at a depth d, seen in FIG. 1, that is sufficiently beneath active area 20 such that metallic contaminants do not encroach therewithin.
What is needed is a method of forming a semiconductor gettering structure that avoids the problems of the prior art. In particular, what is needed is a method of forming a semiconductor gettering structure by ion implantation that minimizes crystal lattice dislocations caused by ion implantation. What is also needed is a method of forming a semiconductor gettering structure that uses less capital and energy cost than that which was used in the prior art.
The present invention relates to the formation of a gettering structure by ion implantation. Formation of a gettering structure is carried out by ion implantation of a material that has an affinity for impurities such as metals. Iron is a typical metal impurity. The ion implantation material may be such elements as oxygen, silicon, germanium, and equivalents. Ion implantation is carried out using high current implantation techniques that are known in the art.
A preferred embodiment of the present invention includes forming a recess in a semiconductive substrate within active areas that border on the recess. The recess is preferably formed by using a reactive anisotropic etching medium. Next, ions are implanted by changing the ion implantation process for a process for a reactive anisotropic etching medium. The inventive method creates separate gettering regions beneath the recess without causing substantial damage within active areas that border on the recess.
An advantage of the method of the present invention is that semiconductor gettering structures are formed without a masking procedure. Additionally, the inventive method can use KeV implantation equipment and processes, although more expensive MeV implantation equipment can also be used. Metallic contaminants will diffuse freely through the semiconductive substrate to any depth. Another advantage of the present invention is that lateral spread of ions that cause damage in the semiconductor substrate during implantation is reduced due to the lower energy KeV implantation of high current implantation as opposed to MeV implantation. Also, the lateral spread of ions during implantation is reduced due to a shallower implantation depth when using the lower energy KeV implantation of high current implantations.
Following implantation and formation of a gettering structure, thermal processing may be carried out in order to induce a lateral spreading or widening of the gettering structure. In some embodiments, it may be desirable that the gettering structure substantially contacts an adjacent gettering structure. Such substantial contact of gettering structures may be accomplished by a variable angle of ion implantation. As such, the method of forming a semiconductor structure includes expanding of the gettering structure by thermal processing.
In another embodiment of the present invention, a dual implantation is carried out. A semiconductor structure is dual implanted whereby a gettering structure and a shallow implantation are formed. The gettering structure has a higher concentration than the shallow implantation. Following dual implantation, a single anneal is carried out to eliminate vacancy clusters and silicon pits that are formed during routine crystal pulling at the region near an upper surface of the semiconductive substrate by injecting interstitials through a self-implantation. Subsequent annealing leaves the gettering structure substantially intact to act as a getterer for further processing and during field use.
The shallow implantation of the interstitial injection migrates during thermal processing to fill crystal originated particles or pits (COPs) within the semiconductive substrate. Where the semiconductive substrate is monocrystalline silicon, implantation of a neutral species such as silicon, germanium, and the like will add interstitials to the semiconductive substrate. During thermal processing, the interstitials will move into the COPs thereby rectifying defects caused during crystal pulling.
An alternate method embodiment combines implantation through a recess in the semiconductive substrate to form a gettering structure with forming a shallow implantation after filling the recess with a dielectric material. In this embodiment, thermal processing to expand the gettering structure is preferably controlled such that the gettering structure does not expand into a diode junction region and thereby cause leakage.
The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrated and not restrictive. These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.